A 76-year-old male is brought from his nursing home to the emergency department for evaluation of altered mental status and confusion. Two days ago he went on a trek to a nearby mountain with a few other residents from the nursing home. He complained of thirst during the return trip and became increasingly combative. Earlier this morning, he developed a seizure while eating his breakfast. His current medication include metformin for diabetes and lisinopril for hypertension. A week ago, his primary care physician added hydrochlorothiazide to his blood pressure regimen. On physical examination, her blood pressure is 78/54 mm Hg, her pulse is 145 beats/min, and her respiratory rate is 24 breaths/min. The patient is disoriented to place and time, and has limited ability to follow commands. Other significant findings include dry mucous membranes, sunken eyes, decreased skin turgor, poor capillary refill and weak thready pulse. Test Result Reference Sodium (Na⁺) 124 136 – 146 mEq/L Potassium (K⁺) 3.8 3.5 – 5.0 mEq/L Chloride (Cl⁻) 97 95 – 105 mEq/L Bicarbonate (HCO₃⁻) 24 22 – 28 mEq/L Glucose 840 mg/dL Fasting: 70 – 100 mg/dL Random, non-fasting: <140 mg/dL Creatinine 2.2 mg/dL 0.6 - 1.2 mg/dL Urea nitrogen 74 mg/dL 7-18 mg/dL pH 7.36 7.35 - 7.45 pCO₂ 36 35 - 45 mm Hg Blood cultures Negative Negative Urinalysis was notable for 2+ glucose and minimal ketones. Computed tomography of the head showed no acute hemorrhages, ischemic changes, or masses. No infiltrates or effusions were visible on plain films of the chest. EKG showed sinus tachycardia. Which of the following is the most likely diagnosis in this patient?
